Priority is claimed to British Patent Application No. 9828304.7, filed Dec. 23, 1998.
The present invention relates to a chain or belt tensioner and more particularly, but not exclusively, to a tensioner of the kind for maintaining tension in a power transmission chain or belt of an internal combustion engine,
In an internal combustion engine endless loop chains are often used to transmit driving power from the crankshaft to the camshaft and/or to auxiliary devices such as water or oil pumps, air conditioning devices etc. The chain travels between a plurality of sprockets each of which is connected to a driving or driven device. Tie tension of a chain can vary as a result of chain and sprocket wear or temperature variations. It is important to ensure that the chain is maintained in a sufficiently tensioned state to prevent noise or slippage of the chain relative to a sprocket.
Known chain tensioners comprise a reciprocal plunger that is biased out of a housing into contact with a tensioning shoe that bears against the chain to impart tension thereto. The biasing force is provided by a combination of a coil spring mounted inside a chamber in the plunger and hydraulic fluid that flows into the chamber from an external source. The hydraulic fluid also serves to damp movement of the plunger in the housing.
Assembly or dismantling of a chain tensioner of this kind to or from the engine can be difficult as a result of the spring force acting between the housing and the plunger. In some designs the components are not held together and the plunger may fall into the engine during initial assembly or installation during servicing of the tensioner. This has led to a separate device being incorporated to hold all the components together. This obviously adds to manufacturing and assembly costs.
In applications where a small tensioning load and therefore a small plunger is required it is not possible to use a conventional coil spring inside a chamber of the plunger and obtain sufficient biasing force.
It is known to mount the biasing coil spring on the outside of plunger and/or housing. Examples are shown in EP-A-0387539 and U.S. Pat. No. 4,973,290 but neither of these designs allows for easy installation or removal from the internal combustion engine.
U.S. Pat. No. 5,700,215 discloses a hydraulic tensioner for a belt or chain that comprises a plunger slidable within a support part that is screwed into a housing. The support part has a spigot that projects from the housing. A coil spring is disposed on the outer surface of the spigot and bears against an end face of the support part and the plunger so as to bias the plunger out of the housing. A stopper screw is provided in the housing and is engageable with part of the plunger to hold the latter in a retracted position during installation or maintenance.
U.S. Pat. No. 4,997,411 described a slack adjuster for a timing belt or chain. The structure of the adjuster is relatively complex with many components. A dynamic plunger is coxially aligned with and slidable relative to a stationary plunger both plungers being received in a sleeve. The dynamic and static plungers are linked by a coil spring that biases the dynamic plunger away from the static plunger and towards the chain to eliminate slack. There is a relatively complex arrangement of check valves for preventing fluid from flowing back into a main chamber in the static plunger when the chain is tensioned and to allow a small amount of fluid and all entrapped air to exit via a passageway. The ends of the coil spring bear against respective flanges, one connected to the dynamic plunger and one with spaced projections connected to the static plunger. The arrangement is not compact and suffers from friction losses ill view of the many components.
According to the present invention there is provided a belt or chain tensioner comprising a housing having a chamber, a plunger slidably displaceable within the chamber and a spring disposed around the outside of the housing and plunger so as to bias the plunger to a position protruding from the housing, the spring being captively connected to the housing by means of at least one coil of the spring being located over a lip on an exterior surface of the housing and captively connected to said plunger, the plunger having an enlarged head that projects from the housing and against which the spring bears.
The chain tensioner can thus be installed into or removed from an internal combustion engine in one piece without the plunger and housing separating under the influence of the spring.
Preferably the coil is received in groove immediately adjacent the lip of housing.
There may be a tapered surface adjacent the head, a coil of the second end of the spring being retained between the head and the tapered surface.
The plunger is preferably solid.